Iraq's draft constitution, to be voted on in a national referendum on October 15, aims to turn Saddam Hussein's old fiefdom into a modern state. But as the al-Aima bridge tragedy demonstrated, Iraq is never far from a state of panic.

It may be that insurgent mortar attacks or an attempted suicide bombing spooked the Shia pilgrims. It may be that, as one Shia leader claimed, "terrorists, Saddamists and radical extremists [are] responsible for what happened". It may have been a terrible accident.

But in the current climate of fear and insecurity, appearances and reality grow confused. Iraq is oil-rich but many Iraqis lack basic facilities. Iraq has had an elected government since January, but democracy has yet to bring stable governance.

George Bush has repeatedly said that America will "finish the job" in Iraq. But the US also sees a constitutional agreement as a key stage in its exit strategy.

"We believe at some point ... you simply have to back off and let the Iraqis step forward," Major General Douglas Lute of US central command said in London last month. Such statements may only increase ordinary Iraqis' insecurities amid the insurgency.

Shia leaders have shown restraint in the face of acknowledged sectarian attacks such as the one in Hilla in February. But while there were no signs yesterday that the al-Aima tragedy would produce a Shia backlash, the trend towards worsening sectarian tensions is marked.

It has been accelerated, paradoxically, by US pressure to agree a constitution which climaxed last week when Shia and Kurdish leaders overruled Sunni objections to the final draft text. "A big signal of divisiveness was that the Shia and Kurds went ahead without consensus in a self-interested way, at the expense of the Sunnis," said Rosemary Hollis, a Middle East expert at Chatham House.

Noah Feldman, a legal adviser to the now defunct coalition provisional authority, warned of a "looming disaster" in which the constitution, even if ratified, died at birth.

"The flawed negotiations, driven at breakneck pace by American pressure to meet an unnecessary deadline, failed to produce an agreement satisfactory to the Sunni politicians," he wrote in the New York Times.

Sunni leaders are alarmed about a federalist formula allowing autonomous regions on the Kurdish model. To their dismay, late in the negotiations leaders of Sciri, one of the biggest Shia parties, demanded autonomy rights for the Shia south.

That unexpected demarche, although opposed by some Shias in Baghdad, left the Sunni minority, which is concentrated in central Iraq, contemplating a future without political power, without a role for ex-Ba'athists, and without a stake in Iraq's oil regions. Some Sunnis say this potentially institutionalises ethnic and religious divisions and puts Iraq's unity at risk. Hence calls for a boycott or a no vote on October 15.

"It makes me sick what I hear from members trying to divide Iraq," Salih al-Mutlak, a Sunni negotiator, told Yaseen al-Rubaie of the Institute for War and Peace Reporting in Baghdad. Those seeking federalism wanted to "slay Iraq", he said.

"Sectarianism has become a self-fulfilling prophecy," Ms Hollis said, fuelled by factors such as the US and British tolerance of Shia and Kurdish militias.

To counter sectarianism, she suggested the best outcome on October 15 might be a big turnout and a no vote. That would indicate public engagement with the political process while boosting resistance to the tyranny of the majority. That, after all, is what the US constitution is all about.